The present invention relates generally to batteries, and, more particularly, to a method and apparatus for identifying the state of charge of a battery during charging or discharging thereof.
A portable, battery power source is required in order to power an electrical energy-consuming device which cannot be positioned proximate to a permanent power supply. As the battery power source is carried with the electrical device which the battery power source powers, it is desirable to minimize both the size and the weight of the battery. For instance, the battery used to power a cellular phone is carried with the phone. In order to minimize the size and weight of the phone and battery package, it is desirable to utilize a small, light-weight battery. However, size and weight minimization limits the energy storage capacity of the battery.
Use of the device powered by the battery discharges the stored energy of the battery, thus depleting the battery of stored energy, necessitating replacement of the battery. The frequency with which the battery must be replaced is, of course, dependent upon the battery capacity, the energy required to operate the electrical device, and the frequency with which the electrical device is operated.
Nickel-cadmium (Ni-Cd) batteries have been developed and are widely used as portable battery power sources for the reason that a Ni-Cd battery, once depleted of stored energy, may be recharged simply by connecting the battery to a suitable charging current source for a period of time. Battery charging apparatus are known to permit such recharging. A nickel-cadmium battery may be recharged as many as five hundred, or more, times, and this battery type is additionally advantageous for the reason that the charging rate at which the battery is recharged may be relatively high (i.e., "fast charging" of the battery is permitted) in order to recharge the battery in a short period of time.
However, when charging a Ni-Cd battery, care must be exercised in order to ensure that fast charging is terminated once the Ni-Cd battery has been fully charged. Otherwise, the battery may become damaged. A user, however, also desires to ensure that the battery is fully charged. Otherwise, soon after the battery is connected to power the electrical device, the battery energy becomes depleted, and the battery must be replaced again.
It is therefore of significant value to accurately detect the amount to which the battery has been charged (i.e., detect the state of charge of a battery) during recharging of the battery.
Several methods and apparatus have been developed in order to detect the state of charge of a battery power source during the recharging thereof. One such method is merely to measure the voltage of the battery during recharging thereof and then to provide a display of the measured voltage levels. One apparatus operative to carry out such a method to detect and display the measured voltage levels of the battery power source is a bar display driver such as a National Semiconductor Corporation LM3914 Dot/bar display driver and a suitable output display device coupled thereto. The MOLICEL (.TM.) evaluation kit, produced by Moli Energy Limited of Burnaby, British Columbia, Canada for MOLICEL rechargeable battery packs, incorporates such a driver. However, this existing method and associated apparatus fails to make adjustments responsive to particular battery characteristics, such as, e.g., battery age, manufacture, and ambient conditions. Inaccurate detections of states of charge result.
Another method for detecting the state of charge of a battery involves measuring the current supplied to the battery during charging of the battery, and measuring current output by the battery during discharging of the battery. By maintaining records of both the amount of current supplied to and output by the battery, the state of charge of the battery is determined. However, this method requires constant monitoring of the battery in order to create and maintain such records. Such constant monitoring is, in many instances, impractical.
A method and apparatus for detecting the actual state of charge of a battery power source which is both accurate and easily obtained is therefore required.
Additionally, an indication of the state of charge of a battery power source during use of an electrical device to which the battery power source is connected, i.e., during discharge of the battery power source, is beneficial in order to provide to a user of an electrical device an indication of the extent to which the battery has been discharged. An early indication that the battery energy levels have been depleted to a critical level allows the user to replace the battery prior to failure of the device. Because a battery characteristic discharge curve is merely the reverse of a battery characteristic charge curve, the battery characteristic discharge curve is also nonlinear. Existing methods and apparatus for detecting and indicating the state of charge of a battery during discharge thereof are similarly inherently inaccurate.